أنا أتشارك 'Lec 7 Mol genetics' معك.pdf

Because learning changes everything. ® BIO222 Molecular genetics 3 Cr. Hrs. = (2 LCT + 0 TUT + 2 LAB + 0 OTH) – SWL = 150 – ECTS = 6 Prerequisite - - - Introduction to Molecular genetics. Regulation of transcription and translation in eukaryotic and prokaryotics....

Because learning changes everything. ® BIO222 Molecular genetics 3 Cr. Hrs. = (2 LCT + 0 TUT + 2 LAB + 0 OTH) – SWL = 150 – ECTS = 6 Prerequisite - - - Introduction to Molecular genetics. Regulation of transcription and translation in eukaryotic and prokaryotics. Genome, Transcriptome, Proteome. Genome structure, stability, and organization. Prokaryotic Genomes. Eukaryotic Genomes. Accessing Genomes. Mapping genomes. Molecular genetics of development. Types of mutations and identification of disease genes. Epigenetics, inherited diseases.  Because learning changes everything. ® Chapter 07 Genetic Transfer and Mapping in Bacteria Genetics: Analysis & Principles EIGHTH EDITION Robert J. Brooker © McGraw Hill LLC. All rights reserved. No reproduction or distribution w ithout the prior w ritten consent of McGraw Hill LLC.  Genetic Transfer and Mapping in Bacteria Eye of Science/Science Source © McGraw Hill 3  Introduction 1 Like eukaryotes, bacteria often possess allelic differences that affect their cellular traits However, these allelic differences (such as different sensitivity to antibiotics) are between different strains of bacteria because Bacteria are usually haploid © McGraw Hill 4  Introduction 2 Bacteria reproduce asexually via binary fission Therefore, crosses are not used in the genetic analysis of bacterial species Rather, researchers rely on a similar phenomenon that is called genetic transfer In this process, a segment of bacterial DNA is transferred from one bacterium to another Bacteriophages (or simply phages) are viruses that infect bacteria Contain their own genetic material that governs their traits © McGraw Hill 5  7.1 Overview of Genetic Transfer in Bacteria Like sexual reproduction in eukaryotes, genetic transfer in bacteria enhances genetic diversity Transfer of genetic material from one bacterium to another can occur in three ways: Conjugation Involves direct physical contact Transduction Involves bacteriophages Transformation Involves uptake from the environment See Table 7.1 © McGraw Hill 6  Mechanisms of Genetic Transfer in Bacteria 1 Three Mechanisms of Genetic Transfer Found in Bacteria Mechanism: Conjugation Description: Requires direct contact between a donor and a recipient cell. The donor cell transfers a strand of DNA to the recipient. In the example shown here, a circular segment of DNA known as a plasmid is transferred to the recipient cell. © McGraw Hill 7  Mechanisms of Genetic Transfer in Bacteria 2 Mechanism: Transduction Description: When a bacteriophage infects a donor cell, it incorporates a fragment of bacterial chromosomal DNA into a newly made phage particle. The phage then transfers this fragment of DNA to a recipient cell, which incorporates the DNA into its chromosome by recombination. © McGraw Hill 8  Mechanisms of Genetic Transfer in Bacteria 3 Mechanism: Transformation Description: When a bacterial cell dies, it releases a fragment of its DNA into the environment. This DNA fragment is taken up by a recipient cell, which incorporates the DNA into its chromosome by recombination. © McGraw Hill 9  7.2 Bacterial Conjugation 1 Genetic transfer in bacteria was discovered in 1946 by Joshua Lederberg and Edward Tatum They were studying strains of Escherichia coli that had different nutritional growth requirements Auxotrophs cannot synthesize a particular nutrient and therefore need it in their growth medium Prototrophs make all their nutrients from components in their growth medium © McGraw Hill 10  7.2 Bacterial Conjugation 2 - - + + + One strain was designated met bio thr leu thi It required one vitamin (biotin) and one amino acid (methionine) It could produce the amino acids phenylalanine, leucine and threonine Another strain was designated met + bio +thr - leu - thi - It required one vitamin (thiamine) and two amino acids, leucine and threonine It could produce the amino acid, methionine, and the vitamin, biotin © McGraw Hill 11  (Figure 7.1) Experiment of Lederberg and Tatum demonstrating genetic transfer during conjugation in E. coli Access the text alternative for slide images. © McGraw Hill 12  Bacteria can Transfer Genetic Material during Conjugation The genotype of the bacterial cells that grew on the plates + + + + + must be met bio thr leu thi Lederberg and Tatum reasoned that some genetic material was transferred between the two strains Either the met - bio - thr + leu + thi + strain gained the ability to synthesize biotin and methionine (bio + met + ) + + - - - Or the met bio thr leu thi strain gained the ability to synthesize threonine and leucine and thiamine (thr + leu + thi + ) The results of this experiment cannot distinguish between these two possibilities © McGraw Hill 13  Conjugation Requires Direct Physical Contact 1 Bernard Davis later showed that the bacterial strains must make physical contact for transfer of genetic material to occur Used an apparatus known as a U-tube It contains a filter at the bottom which has pores that were Large enough to allow the passage of the genetic material But small enough to prevent the passage of bacterial cells © McGraw Hill 14  (Figure 7.2) U-tube apparatus like that used by Davis Access the text alternative for slide images. © McGraw Hill 15  Conjugation Requires Direct Physical Contact 2 Davis placed the two strains in question on opposite sides of the filter Application of pressure or suction promoted the movement of liquid through the filter Samples of bacteria from either side were then placed on growth media that selected for the met + bio +thr + leu +thi + genotype Nothing grew! Davis concluded that the bacteria must make direct physical contact to transfer genetic material © McGraw Hill 16  Conjugation Requires Direct Physical Contact 3 The term conjugation now refers to the transfer of DNA from one bacterium to another following direct cell-to-cell contact Many, but not all, species of bacteria can conjugate However, only certain strains of bacteria can act as donor cells © McGraw Hill 17  F factors Carry Genes that are Required for Conjugation In E. coli certain donor strains contain a small circular piece of DNA termed an F factor (for fertility factor) Strains containing an F factor are designated F+ Those lacking it are F- Genes that play a role in the transfer of DNA: Named tra or trb followed by a capital letter © McGraw Hill 18  (Figure 7.3) Genes on the F factor that play a role during conjugation Access the text alternative for slide images. © McGraw Hill 19  Contact Initiates Conjugation The first step in conjugation is the contact between donor and recipient cells This is mediated by sex pili (or F pili) which are made only by F+ strains These pili act as attachment sites for the F- bacteria Once contact is made, the pili shorten Donor and recipient cells are drawn closer together A conjugation bridge is formed between the two cells The successful contact stimulates the donor cell to begin the transfer process © McGraw Hill 20 © McGraw Hill 21 © McGraw Hill 22  Mechanism of Transfer 1 1. Relaxosome recognizes origin of transfer (DNA sequence); makes a cut in the DNA 2. After cutting, most accessory proteins of the relaxosome are released 3. One protein, relaxase, remains bound to the end of the T-DNA 4. Exporter is a complex of 10 to 15 proteins coded by the F factor that span both inner and outer membranes; pumps DNA/relaxase into recipient cell © McGraw Hill 23  (Figure 7.4a) Mechanism of Transfer 2 Access the text alternative for slide images. © McGraw Hill 24  (Figure 7.4a) Mechanism of Transfer 3 (a) Transfer of an F factor via conjugation Access the text alternative for slide images. © McGraw Hill 25  Mechanism of Transfer Outcome The result of conjugation is that the recipient cell has acquired an F factor Thus, it is converted from an F- to an F+ cell The F+ cell remains unchanged © McGraw Hill 26  Plasmids Plasmid is the general term used to describe extra-chromosomal DNA Most are circular, although some are linear Present in many bacteria and a few eukaryotic species Range in size from a few thousand to 500,000 bp Some, called episomes, can integrate into a chromosome Plasmids have their own replication of origin Replicate independent of bacterial chromosome Depending on ‘strength’ of origin, can exist as single copy or up to 100 copies per cell Plasmids are usually not required for survival Can provide growth advantages to the bacteria © McGraw Hill 27  Five Different Categories of Plasmids 1 1. Fertility plasmids Allow conjugation 2. Resistance Plasmids Known as R factors Contain genes conferring resistance to antibiotics 3. Degradative plasmids Carry genes allowing digestion of unusual substances © McGraw Hill 28  Five Different Categories of Plasmids 2 4. Col-plasmids Contain genes that kill other bacteria 5. Virulence plasmids Carry genes that turn bacterium into pathogenic strains © McGraw Hill 29